Drill spring tension limiting device for floating drilling vessels

ABSTRACT

An apparatus for limiting the tension to which a suspended member can be subjected is disclosed. The apparatus responds to excessive tensile stress by increasing in length at a rate to keep such stress from exceeding a predetermined level. The present invention includes a closed cylinder with a piston disposed therein. Connected to the piston is a piston rod which extends through an opening in one end of the cylinder. The cylinder and piston rod are adapted to connect with a suspended member and a hoisting means. A bypass conduit connects the cylinder chambers by extending from one side of the piston to the other. In the conduit, there is a flow control regulator which actuates when the pressure on one side of the piston reaches a predetermined level, corresponding to a proportional tensile stress, permitting fluid to flow through the conduit in such a volume that the pressure on that side of the piston is maintained at the predetermined level.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to limiting the maximum tension in a string of drill pipe or other member suspended from a floating vessel and which may be subjected to stresses of magnitudes that could cause the pipe to yield or even fail.

2. Description of the Prior Art

As the demand for hydrocarbons has increased, the search for crude oil and natural gas has been extended further out into the sea. With increasing water depths, more and more drilling is conducted from floating vessels. A number of problems are encountered in floating drilling operations as a result of vessel motions caused by wind, waves and currents. Vertical movements of the vessel pose particularly severe problems. For most drilling operations, however, vessel heave is compensated for by the use of slip joints positioned in the suspended tubular members, permitting them to extend rather than become overstressed.

At times, drilling procedures must be performed which render the telescoping joints of little use in alleviating pipe stresses caused by vertical motion. For example, a number of operations require the drilling rig to pull upward on an apparatus or other load which is anchored to the sea floor. This may result from the apparatus being stuck, allowing no vertical movement, or restrained by design, allowing only limited vertical movement. When using the suspended pipe string to pull upward on such apparatus, all of the slip joints will, of course, be fully extended. In such a case, there is a significant risk that the suspended pipe string may be excessively stressed, especially if the floating vessel suddenly heaves upward due to wind or wave action. Not only could the excessive stress cause the suspended pipe string to yield or even fail, but it also could damage the subsea apparatus. Therefore, there exists a need for an apparatus that will limit the maximum tension which can be imposed on a suspended pipe string.

SUMMARY OF THE INVENTION

The present invention is directed to apparatus which alleviates the difficulties outlined above by limiting the amount of stress to which a suspended member can be subjected. The apparatus of the present invention is positioned in the pipe string and will function as any other span of pipe as long as the axial stress in the string is less than the maximum allowable stress preset in the apparatus. When the axial stress increases to the predetermined maximum, the apparatus of the invention begins to increase in axial length, preventing the stress in the pipe string from exceeding its predetermined maximum allowable stress.

The present apparatus includes a hollow cylinder having a slideable piston disposed therein which serves to divide it into two separate chambers. A piston rod is attached to the underside of the piston and extends through and forms a seal with an opening in one of the end walls of the cylinder. A bypass conduit connects the two cylinder chambers. Positioned in the conduit is a means for controlling the flow of fluid between the chambers. When pressure on the underside of the piston reaches a preset level, the flow control means opens to permit a sufficient flow of fluid through the conduit to maintain the pressure on the piston at or below the pressure level corresponding to the maximum allowable stress in the suspended member or the apparatus to which it is connected. The apparatus of the invention is adapted to permit one end to be connected to the drill rig hoisting means and the other to the member to be suspended. Preferably, a return conduit also extends between the two cylinder chambers. The return conduit serves to conduct fluid back to the rod side of the piston, permitting the apparatus to be reset. Means are provided in the return conduit for permitting flow of fluid to the rod side of the piston from the other side during the reset operation, while preventing fluid flow in the opposite direction during normal operation of the stress limiting device. Preferably, the means provided for this purpose is a check valve.

In operation, the stress limiting device of the invention is integrated into the suspended member between the hoisting apparatus and the bottom load. When the hoist pulls upward, it increases the tensile stress in the suspended member and the apparatus of the invention. This in turn causes a proportionate increase in the pressure of the fluid contained in the chamber on the underside or rod side of the piston. When the pressure in the chamber reaches a preset limit, the control means in the conduit opens, allowing the fluid to flow from one side of the piston to the other, permitting the piston to slide relative to the cylinder. In this fashion, the apparatus of the invention keeps the tension in the suspended member from exceeding its predetermined maximum allowable stress. As a result, neither the apparatus on the sea floor nor the suspended member will become overstressed to the point where it will yield or fail.

The present invention will thus be seen to provide a practical system for limiting tension in pipe strings or other members suspended from a floating vessel by preventing the tensile stress from exceeding a predetermined maximum allowable level. The apparatus of the present invention therefore offers significant advantages for operations where suspended members, or apparatus attached thereto, may be subjected to stresses that may cause them to yield or fail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic elevation view, partially in section, of a drilling vessel floating on a body of water and provided with apparatus embodying the present invention.

FIG. 2 is a cross-sectional view of the preferred embodiment of this invention.

FIG. 3 is a cross-sectional view of another embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 depicts a typical offshore drilling vessel 21 and a possible relative location of the apparatus of the present invention. The vessel 21 will be understood to include the normal complement of equipment used in offshore drilling operations, although only a part of this apparatus is shown. Drilling vessel 21 is shown floating on a body of water 10. The stress limiting apparatus of the present invention is identified by numeral 20 and is depicted as suspended directly from the traveling block 22. Traveling block 22 is in turn suspended from crown block 23 situated atop drilling derrick 24. The draw works 25 is connected to the traveling block by hoisting cable 26 which extends over crown block 23. The draw works and block-and-tackle arrangement will be referred to collectively herein as the hoisting means.

As depicted, drill string 27 is attached to and suspended from the lower end of stress limiting device 20. It will therefore be apparent that the stress limiting device is so located as to limit the maximum stress in the drill string suspended therebelow as well as the load to which it is connected. As long as stress limiting device 20 is positioned anywhere beneath the traveling block, it will function to limit tensile stress within the drill string.

Drill string 27 extends downwardly through riser 28, through blowout preventer 13 located on bottom 11 of body of water 10, and through surface casing 14. It is shown to be attached at its lower end to a load 12 positioned in the well bore 15. Load 12 is depicted schematically and represents an actual load which will induce large magnitude stresses in the drill string when it is lifted upward. For example, a load of the type which would occur when the drill string, or apparatus attached thereto, becomes stuck in the wellbore.

FIG. 2 depicts schematically, in section, a preferred embodiment of the present invention. A cylinder 30 is shown which is closed at one end and has an opening 35 in the opposite end wall. A piston 33 is slidably disposed within cylinder 30 and includes an appropriate seal between its exterior and the inner bore of the cylinder. A resilient seal ring designated as 42 is shown, although it will be clear that other types of seal means may be used. Piston 33 divides the cylinder into two independent chambers, designated by numerals 31 and 32. Chamber 32, shown on the underside of the piston, is also referred to herein as the rod side of the piston. Connected to the piston is a piston rod 34. It extends through opening 35 in the end wall of cylinder 30. The space between openings 35 and piston rod 34 is appropriately sealed to prevent leakage therethrough by a resiliant seal ring 43. Of course, other types of seal members may be used.

A bypass conduit 50 connects chambers 31 and 32, providing a flow path for the fluid in cylinder 30 which extends from one side of the piston to the other. Preferably the bypass conduit is a rigid, metallic tubular member since it must withstand high internal pressures. Conduit 50 communicates with chambers 31 and 32 disposed on opposite sides of the piston through ports 36 and 37 extending through the cylinder wall. Ports 36 and 37 are shown located in the side walls of cylinder 30, but it will be appreciated that they could be located in the end walls of cylinder 30 as well. Preferably, they are positioned close to the ends of the cylinder so that the two chambers will be in fluid communication throughout the full stroke of the piston.

Located in conduit 50 is a means for controlling flow of fluid therethrough shown as back pressure regulator 51. This control means normally prevents flow of fluid from one side of the piston to the other; however, it allows fluid flow whenever the pressure in the chamber beneath the piston reaches the preset maximum allowable pressure level. Since the fluid pressure under the piston is proportional to the tensile stress in the suspended member, limiting the maximum fluid pressure necessarily limits the maximum tensile stress to which the suspended member will be subjected. When back pressure regulator 51 opens, it allows interchamber flow through the bypass conduit at a rate such that the pressure beneath the piston does not exceed the maximum allowable pressure. If the pressure should drop below the maximum allowable pressure, back pressure regulator 51 closes, halting the flow of fluid. It will be understood, of course, that control means other than a back pressure regulator also could be effectively used to perform this function.

Although the bypass conduit and control means are shown situated on the exterior of cylinder 30, they also could be disposed entirely within it. In such an arrangement, the bypass conduit could comprise a passageway extending across piston 33. Similarly, the control means 51 would be positioned within or attached to the piston and would be capable of controlling the flow of fluid through the bypass conduit in the same manner as the externally located back pressure regulator.

It nevertheless, is preferable to locate the conduit and control means outside the cylinder to provide easy access to the bypass circuit. This not only enables the pressure setting of the control means to be quickly reset to conform to the current requirements of the specific situation, but also simplifies the service and repair of the regulator and bypass system.

A return conduit 60 connects opposed chambers 31 and 32 by means of full opening connections to conduit 50 at points 40 and 41, thereby effectively circumventing the back pressure regulator. Preferably, conduit 60 is a rigid metal tube capable of withstanding the internal pressures to which it will be subjected. Preferably ports 36 and 37 are situated such that the return conduit provides a flow path throughout the entire reset stroke of piston 33, i.e., when the volume of chamber 31 is being reduced and that of chamber 32 is being increased.

Located in the return conduit is a flow control means depicted in the drawing as check valve 61. Check valve 61 allows fluid to flow from one chamber to the other whenever the apparatus is being reset as is required to restore the piston to its initial position. The check valve prevents flow through the return conduit when tension limiting apparatus 20 is being used to limit tension in a suspended member. It will be understood that many other types of valves also could be used effectively for this purpose, including manually, hydraulically, or electrically actuated valves. Although return conduit 60 and flow control means 61 could be completely contained inside the cylinder, e.g., within the piston, the preferred location is outside the cylinder to facilitate service and repair.

As depicted, the upper end of tension limiting apparatus 20 is connected to the hoisting means by the bales of a standard elevator connected to a tool joint designated by numeral 70. The piston rod 34 is shown connected to the suspended member by a standard tool joint pin 71. It will be appreciated, however, that, alternatively, other standard connecting means could be used. It should be noted that the apparatus of this invention 20 will perform just as effectively inverted, i.e., if piston rod 34 were attached to the hoisting means and cylinder 30 attached to the suspended member. Similarly, the stress limiting device of the invention can be positioned in the suspended member well below the hoist means.

FIG. 3 is a schematic sectional view of another embodiment of the present invention. The distinction between the embodiments of FIG. 3 and FIG. 2 is the arrangement of the return conduit and return flow control means. Instead of attaching the return conduit as a loop on bypass conduit 50 as shown in FIG. 2, an independent return conduit, is provided which connects chambers 31 and 32 through ports 38 and 39 extending through the walls of cylinder 30. It will further be noted that either bypass conduit 50 or return conduit 60, alternatively, could be independently contained within piston 33.

Although not shown in the drawings, as least one sealable filler port must be provided somewhere in the cylinder or conduit walls to permit the apparatus to be filled with fluid. An incompressible fluid, preferably a hydraulic fluid, should be utilized. In addition, it is preferable that bypass conduit 50 and return conduit 60, when located external to cylinder 30, be enclosed by protective caging to prevent accidental damage during handling. As an alternative to protective caging, the conduits could form an integral part of the cylinder wall.

At the beginning of normal operations, it is desirable to have piston 33 positioned within cylinder 30 such that chamber 31 above the piston is of minimal volume and chamber 32 beneath the piston is of a maximum volume. This orientation allows the maximum stroke of piston 33. As the hoisting means lifts upward, the apparatus of the present invention 20 and the suspended member, drill string 27 as illustrated, are subjected to increasing axial stresses. As long as the axial tensile stress in drill string 27 is less than the predetermined maximum allowable stress, no fluid will be allowed to flow between the two chambers. Therefore, the apparatus of this invention 20 will perform as a rigid segment of drill string 27.

When the tensile stress in drill string 27 reaches the predetermined maximum allowable level, back pressure regulator 51 will open to allow fluid to flow through bypass conduit 50 extending between chamber 32 and chamber 31. Control means 51 regulates the rate of flow as required to allow the fluid to flow at such a rate that the stress in drill string 27 will not exceed the predetermined maximum allowable stress. It will be clear that this fluid flow will cause the stress limiting apparatus of the present invention 20 to elongate, preventing the stress in drill string 27 from exceeding the predetermined maximum allowable stress. The desired maximum allowable stress in the suspended member is set by setting back pressure regulator 51 to open when the fluid pressure in chamber 32 reaches a maximum allowable pressure corresponding to that stress level.

Preferably, a warning alarm system, shown as number 80 in the drawings, is incorporated into the apparatus of the present invention 20 to signal when the maximum allowable stress in the suspended member is attained. The alarm may be actuated by a limit switch which senses the motion of piston 33 or piston rod 34 relative to cylinder 30 or by a pressure switch actuated by the pressure in chamber 32. Alternatively, it could be actuated by the opening of back pressure regulator 51 or by direct measurement of the stress in the suspended member.

When it is desired to reset the apparatus of the present invention 20 to minimize the volume of chamber 31 and maximize the volume of chamber 32, it normally is necessary to place an axial compressive stress across the apparatus 20; however, the amount of force will be minimal since the slightest compressive pressure will open check valve 61, allowing fluid to flow through return conduit 60 from one side of the piston to the other. It should be noted that a pump could be placed in the return conduit to pump fluid from one chamber to the other when it is desired to reset the apparatus. This arrangement would eliminate the necessity of axially compressing the device. 

I claim:
 1. In a floating drilling system including a floating drilling vessel having a drilling derrick, a crown block disposed within the derrick, a traveling block which is suspended below the crown block by a hoisting cable which extends through the traveling block and over the crown block, and a string of drill pipe which is suspended by and below the traveling block, an improved apparatus for limiting tension in the drill pipe which is interconnected between the traveling block and the drill pipe which comprises:a. a cylinder closed at one end and having an opening at the other end; b. a piston disposed within said cylinder, slideable therewithin and forming a slideable seal with the inner wall thereof; c. a piston rod connected to said piston, extending through said opening in the end of said cylinder and forming a slideable seal with the wall of said cylinder at said opening; d. a bypass conduit extending from one side of the piston to the other; e. a return conduit extending from one side of the piston to the other and a means in said return conduit for permitting flow therethrough to the rod side of the piston from the other side and preventing fluid flow in the opposite direction; f. means in said bypass conduit for controlling the flow of a fluid therethrough from the rod side of said piston to the other, said means adapted to actuate when pressure on the rod side of the piston reaches a predetermined level to permit sufficient fluid to flow through the said conduit to maintain such pressure at the predetermined level, said means otherwise preventing flow therethrough when said pressure is below said predetermined level; and g. means for connecting said piston rod and said cylinder to the drill pipe and the traveling block.
 2. The improved apparatus of claim 1 wherein said bypass conduit is positioned externally of said cylinder and extends from a port through the wall of said cylinder adjacent one end to a port through the wall of said cylinder adjacent the opposite end.
 3. The improved apparatus of claim 1 wherein said means for controlling the fluid flow is a backpressure regulator.
 4. The improved apparatus of claim 1 wherein the return conduit is positioned externally of said cylinder and extends from a port through the wall of said cylinder adjacent one end to a port through the wall of said cylinder adjacent the opposite end.
 5. The improved apparatus of claim 1 wherein said return conduit is connected to the said bypass conduit at two points and forms a loop around the said means for controlling fluid flow through said bypass conduit so as to provide a path for fluid flow therearound.
 6. The improved apparatus of claim 1 wherein said means for permitting the flow through said return conduit is a check valve. 